What system are you trying to register them with, CME or CUCM?
Do you have proper option 150 on the dhcp scope? Are the phones getting IP address in proper VLAN, and are they getting the TFTP server address? If so, are they configured in the system or are you using auto registration? What messages do you see on the phone under system messages log?
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.